Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Mountain whitefish Prosopium williamsoni are widely distributed throughout western North America, including many streams and lakes in the western Wyoming (Scott and Crossman 1973; Baxter and Stone 1995). Mountain whitefish are an important ecological and recreational component of streams and lakes in Wyoming, such as the upper Green River. Historically, mountain whitefish were likely one of the most abundant sport fishes in the Intermountain West, including states such as Montana (Brown 1952) and Wyoming. Mountain whitefish prey on a variety of aquatic taxa and terrestrial insects (Brown 1971; Pontius and Parker 1973; Scott and Crossman 1973). Therefore, mountain whitefish often compose a large portion of the biomass present within streams and lakes and likely contribute substantially to ecosystem processes (e.g., nutrient cycling). Additionally, mountain whitefish contribute to the diets of avian predators such as osprey (Van Daele and Van Daele 1982) and terrestrial predators such as river otters (Melquist and Hornocker 1983) and mink. Mountain whitefish can be an indicator species with respect to the effects of global climate change on river water temperatures. For example, upper lethal temperatures for mountain whitefish have not been defined, but weekly mean water temperature tolerances were lower for mountain whitefish than rainbow trout and brown trout (Eaton and Scheller 1996). Finally, mountain whitefish are a native species that makes a substantive contribution to angler recreation in streams of western Wyoming, including the upper Green River. Status and trends in Wyoming’s mountain whitefish populations are only perceived with a limited baseline for comparison, which is currently being established. A recently completed project by the Aquatic Assessment Crew of the Wyoming Game and Fish Department studied the sampling techniques for adult mountain whitefish (Edwards 2014). In addition, the project evaluated the reliability of data collected on mountain whitefish populations for assessing trends in populations. Despite the putative abundance of mountain whitefish in the water bodies of western Wyoming (Baxter and Stone 1995) and their availability as a sport fish (Baxter and Stone 1995; Scott and Crossman 1973), relatively little is known about the ecology of mountain whitefish in Wyoming. Fish population monitoring programs in Wyoming have only recently targeted mountain whitefish due to a lack of methodology. Therefore, available data are difficult to decipher and potentially unreliable for identifying long-term trends. This is also the case in neighboring states such as Montana (Mountain Whitefish Summit 2009). Many of Wyoming’s populations appear stable, however, at least some mountain whitefish populations nearby are showing signs of decline (Mountain Whitefish Summit 2009). In Montana, mountain whitefish numbers have declined in Hebgen Lake since the early 2000s (Mountain Whitefish Summit 2009). Trend data are not available for the Madison River, but the Madison River Foundation and Montana Fish, Wildlife and Parks indicated concern over an apparent decrease in mountain whitefish. Thus, the Madison River Foundation, Trout Unlimited, NorthWestern Energy, and Montana Fish, Wildlife & Parks funded a research project identifying life-history characteristics, habitat requirements, and factors potentially limiting recruitment of mountain whitefish in the Madison River (Boyer et al. 2017a). Interestingly, mountain whitefish moved downstream to spawning locations in the Madison River and it appeared that channel braiding was important large-scale characteristic in spawning site selection. Maturation and fecundity in the Madison River population were similar to other populations, and reproductive development appeared normal, thus factors influencing recruitment probably occur post spawning. Age-0 fish were associated with silt-laden backwater and eddy habitats. Furthermore, age-0 mountain whitefish were most efficiently sampled with seines. The Boyer et al. (2017a) study added a considerable amount of knowledge to mountain whitefish ecology in the Madison River, Montana. Spawning characteristics and early life history are major gaps in the research of mountain whitefish ecology. Effective methodology for sampling juvenile whitefish is a concern of field biologists that is untested in many systems. It is probable that Wyoming could experience declines in mountain whitefish populations and would be best prepared to address potential issues by filling these data gaps. Declines in mountain whitefish populations were documented in a few lakes of the upper Green River drainage of Wyoming to date, and were attributed primarily to the addition of non-native piscivores. Riverine mountain whitefish populations appear to be robust throughout the state, but trends have yet to be identified. Factors limiting production of mountain whitefish may occur at various life-history stages; however, spawning success, abundance of spawning individuals, and early life-history survival can have a strong influence on year-class strength and population abundance. Therefore, identifying large-scale spawning locations and movement, age structure, age-at-maturity, spawning periodicity, and the distribution and habitat requirements of age-0 mountain whitefish will provide a strong foundation for understanding the population dynamics of mountain whitefish in large Wyoming rivers. Knowledge regarding the ecology of mountain whitefish is necessary before we can understand the mechanisms for potential population declines. Large-scale movement and habitat use by, and the spatial and temporal distribution of age-0 mountain whitefish within streams, is largely unknown. The most recent study conducted on age-0 mountain whitefish in the Madison River, Montana found that age-0 mountain whitefish were most efficiently sampled by seining (Boyer et al. 2017b). It is not clear whether the efficiency of seining is consistent among waterbodies throughout the species range. Therefore, evaluating appropriate methods for sampling age-0 mountain whitefish are warranted for varying waterbodies, and application of appropriate sampling methods to characterize the distribution of age-0 mountain whitefish in a large Wyoming river system may help identify limiting factors for mountain whitefish recruitment.